1. Technical Field
The present disclosure relates to integrated image sensors with a curved surface and, more specifically, to a method for manufacturing such image sensors.
2. Discussion of the Related Art
An integrated image sensor comprises a large number of elementary cells or pixels which are arranged in an array at the surface of a semiconductor substrate. The surface of the image sensor is generally planar. To focus the image towards the sensor, an optical system is placed in front of the image sensor.
FIG. 1 schematically illustrates a planar image sensor and an optical focusing system enabling to acquire a high-quality image.
FIG. 1 shows an image sensor 10 and the optical system 12 associated therewith. As can be seen in the drawing, in order for image sensor 10 to properly detect an image placed at infinity and slightly inclined with respect to the optical axis of the system, optical system 12 integrates a large number of elementary optical devices 14. In the shown example, optical system 12 comprises six elementary optical devices 14 (for example, lenses), which will not be further detailed herein.
Elementary optical devices 14 enable focusing all the incident light rays passing through a diaphragm 16 towards image sensor 10, and also to correct aberrations due to the fact that image sensor 10 has a planar surface.
Indeed, images at infinity, capable of being detected by an image sensor, are naturally focused on a curved surface and not on a plane. The optical system associated with the sensor of FIG. 1 is thus particularly complex, to take into account the fact that the image sensor is planar.
Image sensors with a curved surface are thus needed.
FIG. 2 schematically illustrates the optics associated with an image sensor having a general curved surface.
In FIG. 2, an image sensor 20 of even curved surface having its center of curvature located on the sensor illumination side is schematically illustrated. An optical system 22 enables to focus all the incident light rays passing through a diaphragm 16 towards image sensor 20. As illustrated in FIG. 2, due to the curvature of sensor 20, optical system 22 is less complex than optical system 12 illustrated in FIG. 1, and only comprises three elementary optical devices 24.
To obtain an image sensor having a curved surface, it has been provided to form image sensor pixels at the surface of a thin membrane, and to then position the thin membrane on a support of curved shape. To improve the deformation capacity of the membrane, a structuration thereof may be provided, at the end of the process, around blocks formed of a large number of pixels. However, membrane structurations performed by etching at the end of the manufacturing process have low-quality interfaces which cannot be restored, wall restoring methods being performed at high temperatures associated with processings of ion implantation and/or wall oxidation type. This implies that sensor pixels close to the etched regions during the structuration may be impossible to use.
Another disadvantage of known methods is that they provide a step of installation of a flexible membrane onto a curved support of adapted shape. This necessitates a large number of manufacturing steps, and thus a non-negligible manufacturing cost.
Thus, there is a need for method for manufacturing an integrated image sensor having a curved surface, which overcomes all or part of the disadvantages of known methods.